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Highly Resonant Wireless Power Transfer: Safe, Efficient, and Over Distance

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Highly Resonant Wireless Power Transfer: Safe, Efficient, and Over Distance Highly Resonant Wireless Power Transfer: WiTricity Safe, Efficient, and over Distance Dr. Morris Kesler WiTricity Corporation ©WiTricity Corporation, 2013 WiTricity Highly Resonant Wireless Power Transfer: Safe, Efficient, and over Distance Introduction Driving home from the airport, Marin noticed his new smart phone was low on battery once again. Its HD display, and apps using GPS, Bluetooth, and LTE/4G data communications conspired to drain the battery quickly. Without looking, he dropped his phone into an open cup-holder in the center console. Hidden several centimeters below the console, a wireless power source sensed the presence of the phone, and queried the device to determine whether or not it was wireless power enabled. The phone gave a valid response and configured itself for resonant wireless power transfer. Under the console, the source electronics turned on and began charging the phone wirelessly—with no need for a charging cradle, power cord, or especially accurate placement of the phone. Marin relaxed when he heard the recharging chime and focused his attention on the road ahead. After exiting the highway, Marin was surprised to see that the price of gasoline had climbed to over $4.00 per gallon, as it had been months since he had last filled his tank of his new car-- a wirelessly charged hybrid electric vehicle. Since installing a wireless 3.3 kW charger in his home and office garage, his car’s traction battery was fully charged every morning before work and every evening as he began his commute home. As Marin’s car silently pulled into his driveway, it communicated with the wireless charger in his garage. The wall mounted charger electronics ran through its diagnostics and sent a low-power pulse to the mat on the garage floor. Sensors in the mat confirmed it was safe to begin charging. As Marin drove into the garage, he simply parked his car as usual. The resonant charger had enough positioning tolerance that it would work without needing any special parking procedures. Marin smiled upon the realization that he no longer had to recharge or refuel two of his most important high tech devices, his smart phone and his hybrid vehicle. Highly resonant wireless ©WiTricity Corporation, 2013 pg. 2 WiTricity power transfer had succeeded to make these essential products more available, convenient, and reliable. Although the story above is fictitious, the wireless power technology described is very real. This article explores the advances in wireless power technology enabled by the use of highly resonant wireless power transfer, how those advances are being applied across a broad spectrum of applications, and how they address the safety concerns in typical applications. Background The idea of transmitting power through the air has been around for over a century, with Nikola Tesla’s pioneering ideas and experiments perhaps being the most well-known early attempts to do so [1]. He had a vision of wirelessly distributing power over large distances using the earth’s ionosphere. Most approaches to wireless power transfer use an electromagnetic (EM) field of some frequency as the means by which the energy is sent. At the high frequency end of the spectrum are optical techniques that use lasers to send power via a collimated beam of light to a remote detector where the received photons are converted to electrical energy. Efficient transmission over large distances is possible with this approach; however, complicated pointing and tracking mechanisms are needed to maintain proper alignment between moving transmitters and/or receivers. In addition, objects that get between the transmitter and receiver can block the beam, interrupting the power transmission and, depending on the power level, possibly causing harm. At microwave frequencies, a similar approach can be used to efficiently transmit power over large distances using the radiated EM field from appropriate antennas [2]. However, similar caveats about safety and system complexity apply for these radiative approaches. It is also possible to transmit power using non-radiative fields. As an example, the operation of a transformer can be considered a form of wireless power transfer since it uses the principle of magnetic induction to transfer energy from a primary coil to a secondary coil without a direct electrical connection. Inductive chargers, such as those found commonly in electric ©WiTricity Corporation, 2013 pg. 3 WiTricity toothbrushes, operate on this same principle. However, for these systems to operate efficiently, the primary coil (source) and secondary coil (device) must be located in close proximity and carefully positioned with respect to one another. From a technical point of view, this means the magnetic coupling between the source and device coils must be large for proper operation. But what about going over somewhat larger distances or having more freedom in positioning the source and device relative to each other? That’s the question that a group at the Massachusetts Institute of Technology asked themselves. They explored many techniques for transmitting power over “mid-range” distances and arrived at a non-radiative approach that uses resonance to enhance the efficiency of the energy transfer (see Physics of Highly Resonant Power Transfer for details) [3]-[6]. High quality factor resonators enable efficient energy transfer at lower coupling rates, i.e., at greater distances and/or with more positional freedom than is otherwise possible (and therefore, this approach is sometimes referred to as “highly resonant” wireless energy transfer or “highly resonant” wireless power transfer (HR-WPT)). The MIT team demonstrated the highly resonant technique using a magnetic field to transfer energy over a mid-range distance of 2 meters, and an industry was born. In some instances, this technology is also referred to as “magnetic resonance”, and it is often contrasted to “induction” for its ability to efficiently transfer power over a range of distances and with positional and orientational offsets. Since that initial demonstration, the use of HR-WPT, or magnetic resonance, has enabled efficient wireless energy transfer in a wide range of applications that was not possible before. System Description Across an application space that spans power levels from less than a watt to multiple kilowatts, a wireless energy transfer system based on HR-WPT often has a common set of functional blocks. A general diagram of such a system is shown in Figure 1. ©WiTricity Corporation, 2013 pg. 4 WiTricity Figure 1: Block diagram of a wireless energy transfer system. Progressing from left to right on the top line of the diagram, the input power to the system is usually either wall power (AC mains) which is converted to DC in an AC/DC rectifier block, or alternatively, a DC voltage directly from a battery or other DC supply. In high power applications a power factor correction stage may also be included in this block. A high efficiency switching amplifier converts the DC voltage into an RF voltage waveform used to drive the source resonator. Often an impedance matching network (IMN) is used to efficiently couple the amplifier output to the source resonator while enabling efficient switching-amplifier operation. Class D or E switching amplifiers are suitable in many applications and generally require an inductive load impedance for highest efficiency. The IMN serves to transform the source resonator impedance, loaded by the coupling to the device resonator and output load, into such an impedance for the source amplifier. The magnetic field generated by the source resonator couples to the device resonator, exciting the resonator and causing energy to build up in it. This energy is coupled out of the device resonator to do useful work, for example, directly powering a load or charging a battery. A second IMN may be used here to efficiently couple energy from the resonator to the load. It may transform the actual load impedance into ©WiTricity Corporation, 2013 pg. 5 WiTricity an effective load impedance seen by the device resonator which more closely matches the loading for optimum efficiency (Equation 5). For loads requiring a DC voltage, a rectifier converts the received AC power back into DC. In the earliest work at MIT, the impedance matching was accomplished by inductively coupling into the source resonator and out of the device resonator [3]. This approach provides a way to tune the input coupling, and therefore the input impedance, by adjusting the alignment between the source input coupling coil and the source resonator, and similarly, a way to tune the output coupling, and therefore the output impedance, by adjusting the alignment between the device output coupling coil and the device resonator. With proper adjustment of the coupling values, it was possible to achieve power transfer efficiencies approaching the optimum possible efficiency (Equation 6). Figure 2 shows a schematic representation of an inductive coupling approach to impedance matching. In this circuit M g is adjusted to properly load the source resonator with the generator’s output resistance. The device resonator is similarly loaded by adjusting ML , the mutual coupling to the load. Series capacitors may be needed in the input and output coupling coils to improve efficiency unless the reactances of the coupling inductors are much less than the generator and load resistances. M g M ML Rg LS LD Vg Lg CS CD LL RL RS RD Input Coupling Source Device Output Coupling Coil Resonator Resonator Coil Figure 2: Schematic representation of inductively coupling into and out of the resonators. ©WiTricity Corporation, 2013 pg. 6 WiTricity It is also possible to directly connect the generator and load to the respective resonators with a variety of IMNs. These generally comprise components (capacitors and inductors) that are arranged in “T” and/or “pi” configurations. The values of these components may be chosen for optimum efficiency at a particular source-to-device coupling and load condition (“fixed tuned” impedance matching) or they may be adjustable to provide higher performance over a range of source-to-device positions and load conditions (“tunable” impedance matching).
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